My sister is turning 15 next month. She got her first period about 2-3 years ago, and it's still not on track. She will go months and months without a period. She has no schedule either. They used to hurt her but now she doesn't even feel them coming. Is it normal that she doesn't have a cycle by now?

Secondly, it is not uncommon for some women to never have regular periods. Some women go through a life time without knowing when their next period will come.
If she is worried, it will do no harm having a talk with a doctor but right now, I don't see it as much as a problem. I would wait another couple of years and if the problem still persists then it may be time to go for a check up.